There are three broad functions of a fish tank filter – biological, mechanical and chemical.
Many people may focus on the first issue and be unaware of the others, as it is the only visible form, but the other two are vital in keeping your fish alive.
There are filters available which focus on just one or two issues, and some which will cope with Triple issues. You should ensure all three bases are covered.
Fish produce ammonia with their gills as they breathe, which is biological waste. They also produce dirt from bodily functions just like any other animal. Despite being naturally occurring, this ammonia is actually quite toxic to fish, especially if left for a while. Even low levels can cause issues such as eye, kidney and liver problems. Filters turn ammonia into nitrate which is less harmful but will still need to be removed with water changes to avoid algae growth
You may need a mechanical filter to remove particles such as uneaten food, dust and general dirt from the water. Debris can make the water cloudy, and cause issues such as mould so need to be cleared regularly. This will need to be removed via a media such as a sponge or pad which captures the particulate
Water can also contain impurities such as hard metals or minerals, and while you should try to get this to the perfect pH level before filling the tank, it needs to be maintained. Activated carbon filters will be needed to stay on top of this
Types Of Fish Aquarium Filter
Tank filters can come in very forms, usually separated by where they are situated when in use. But some forms are better at certain filtration types than others, and others are only good for one type of water.
Also known as box filters or corner filters. These can be some of the smallest filters around, good for micro tanks under 20 gallons with maybe just one or two fish in, but can also cope with tanks over 100L. They are also relatively inexpensive.
Most can be loaded with multiple forms of filter media, so will clear all three filter needs but others may be restricted to one or two, meaning you need an extra filter. Suction cups keep them fastened, and you may also need a pump to help with water flow movement if the suction is weak
These look similar to an internal model, except the filter sits on the outside of the tank because of its size. These filters can cope with larger capacities, often over 40 gallons. They draw water in via a siphon method and commonly have three filter sections which can usually deal with all three filtration needs. Some may be poor at biological filtering as they technically don’t use air or gravity to actually move the water and collect it all, so an extra pump could be a good idea too
Hang On Back/Power
HoB filters are very popular because they literally hang over your tank, so are simple to fit with no extra tubing. This means they don’t take up space either inside or out and are always easy to access. Most can deal with all three filtration types, cleaning the water before releasing it back into the tank, although they excel with mechanical filtering in particular
As you’d imagine, these are grates which rest under the base level of gravel in your tank. They draw water through the holes on the top and often focus on just mechanical filtration as they collect the debris which falls. Water is then pushed out through the airlift tubes.
There is no filter to change, making them very cheap and easy to run. Some do have a carbon filter but the effectiveness is limited and they are often just removed, so under gravel filters are not meant to maintain the tank by themselves
AKA a trickle filter, sump or shower filter. They get their name from the fact that part is exposed to the tank water, and also to a large amount of air.
Water is siphoned into the internal part and then back out and passed over a biological material source contained in the filter chamber. Bio Balls are the most popular material. This makes them great for biological filtration as they have good access to oxygen, but a bit poorer for the other forms.
They are most commonly used in saltwater aquariums as they need more help producing beneficial bacteria, and are best bought from knowledgeable dedicated fish pet stores because they are hard to set up and you need to buy the precise size for your tank
This all depends on the type of filter you buy, the size of your tank, as well as the ease of maintaining it. You can buy a basic internal filter for as little as £10, and wet/dry or external options can reach closer to £100.
Be aware that you may find those which are a bit more expensive are better at the overall job, and easier to clean and maintain.